A frequency mixer is used for mixing an high frequency input signal RF with a local signal Lo and producing intermediate frequency (IF) signals of sum and/or difference between the input and local signal frequencies. A frequency mixer in a microwave band or a millimeter wave band utilizes a nonlinear characteristic of, for example, diodes and/or transistors. There are several types of frequency mixer, such as an unbalanced mixer, a single balanced mixer and a double balanced mixer. This invention is directed to a single balanced mixer.
In the past, the inventor of this invention has proposed a single balanced mixer for microwave and millimeter wave bands in a Japanese Patent Application No. 11-7252 filed Sep. 22, 1999 owned by the same assignee of this invention. An essential structure of this prior invention is shown in FIG. 5. It should be noted that this prior invention is not in the public domain and thus not the prior art against the present invention.
In the example of FIG. 5, a wide band single balanced mixer includes field effect transistors (FET) as mixing elements. A structure and operation of the single balanced mixer of FIG. 5 will be explained in the following. The single balanced mixer includes a 180° hybrid coupler 2, terminal resistors R1 and R2, field effect transistors FET3 and FET4 as mixing elements, RF frequency λ/4 microstrip line couplers 12 and 13, IF filters 8 and 9, and a 180° hybrid coupler 10.
In operation, a local signal Lo is supplied to an input terminal of the 180° hybrid coupler 2 whose other input terminal is connected to the terminal resistor R1. The 180° hybrid coupler 2 produces two local signals with the same amplitude and opposite phase (0° and 180°) to one another at its output terminals. The two local signals with opposite phase (0° and 180°) are respectively applied to the gates D of the transistors FET3 and FET4, thereby turning on or off the transistors FET3 and FET4.
An input RF signal is separated into two signals of the same amplitude and same phase at a T-junction near the RF input terminal in FIG. 5. One of the two separated RF signals is applied to the drain D of the transistor FET3 through the λ/4 microstrip line coupler 12, and the other separated RF signal is applied to the drain D of the transistor FET4 through the λ/4 microstrip line coupler 13.
Then, the local signals Lo of the opposite phase and same amplitude and the input RF signals of the same phase and same amplitude are mixed with one another by the frequency mixer formed of the transistors FET3 and FET4, thereby frequency converting the input RF signals into IF (intermediate frequency) signals of opposite phase (0° and 180°) and same amplitude which are produced at the drains D of the transistors FET3 and FET 4.
The IF signals of the opposite phase (0° and 180°) and same amplitude are respectively filtered to a desired IF frequency by the IF filters 8 and 9. Typically, the IF filters 8 and 9 are band-pass filters for allowing signals of only selected frequencies to pass therethrough. Moreover, the two IF signals filtered by the IF filters 8 and 9 are combined in the same phase into one IF signal by the 180° hybrid coupler 10. As shown in FIG. 5, the 180° hybrid coupler 10 produces the resultant IF signal at one output terminal while its other terminal is terminated by the terminal resistor R2.
The more detailed explanation is given here regarding the λ/4 microstrip line couplers 12 and 13 in the wide band single balanced mixer in the prior invention of FIG. 5. The purpose of the λ/4 microstrip line couplers 12 and 13 is to provide the input RF signals to the mixer transistors FET3 and FET4 while isolating the IF signal from the RF signal. For this purpose, each of the λ/4 microstrip line couplers 12 and 13 has a length which is one fourth of the wave length of the input RF signal.
At each drain of the transistors FET3 and FET4, there exist IF signals produced by the frequency conversion and the RF signals through the λ/4 microstrip line couplers 12 and 13, respectively. Therefore, in order to prevent the frequency converted IF signals at each drain D of the transistor FET3 and FET4 from leaking into the RF signal input, the λ/4 microstrip line couplers 12 and 13 are designed that it has a low impedance for the RF frequency and a high impedance for the IF frequency.
FIG. 6 shows an example of structure of the λ/4 microstrip line couplers 12 and 13 formed on a planar substrate. Each of the λ/4 microstrip line couplers 12 and 13 is a coupler made of two parallel strip lines having a pattern length of λ/4 of the RF signal wave length with a pattern width W and a pattern gap G. The pattern length λ/4 of the coupler is roughly determined, for example, based on the wave length λ of center frequency of the anticipated frequency bandwidth of the RF signal. Then, an optimum value of the pattern length λ/4 will be determined by fine tuning the length in view of the desired overall frequency characteristics of the single balanced mixer.
When increasing the pattern gap G, the isolation between the IF signal and the RF signal increases which means that the degree of coupling between the IF signal and the RF signal decreases. Further, when increasing the gap, an insertion loss also increases, which degrades the transmission characteristics of the overall mixer. Therefore, it is necessary to optimize the pattern gap G to fit the design specification. Similarly, the insertion loss decreases when increasing the pattern width W which means that a bandwidth of the coupler is broadened, which degrades the IF signal separation (selectivity) . Therefore, it is necessary to optimize the pattern width W to fit the design specification.
In the single balanced mixer, a degree of isolation between the IF signal and the Rf signal is an important factor in determining the mixer performance. FIG. 7 shows a frequency (isolation) characteristics of the λ/4 microstrip line couplers 12 and 13 of FIG. 6. In this example, when attenuation (insertion loss) for the RF frequency in the range between 50 GHz and 80 GHz is one (1) dB, an attenuation (isolation) level for the IF frequency of 30 GHz is 15 dB. The frequency characteristic of the λ/4 microstrip line couplers 12 and 13 of FIG. 6 shows a better degree of isolation between the IF signal and the RF signal compared to a conventional capacitance coupler which typically has an attenuation (isolation) level of about 7 dB.
However, in an application such as in high precision test instruments, further improvement in the single balanced mixer are desired such as increase in the isolation between the IF signal and the RF signal, decrease in the insertion loss, and increase in the conversion efficiency. As describe in the foregoing, the wide band single balanced mixer of FIGS. 5-7 using the λ/4 microstrip line couplers, is insufficient to meet the requirements in such applications.